Like rank in a shmup, does the game get harder (more enemies, thicker bullet swarms, etc) the more mansion upgrades you buy?

Or is it just an indicator of your strength relative to the enemies you're taking on?

As far as I can tell after substantial progress, no. But there are various ways to intentionally tweak the challenge and rewards of the various areas for your farming pleasure.

I have a vague suspicion it may actually affect the prices of castle upgrades, but I could be way off the mark with that. I don't really understand how the levels work, despite being level 60-something.

Your level equals the number of manor upgrades, and the price of all other upgrades definitely does crank up each time you buy one.

I've tried wiping my save file a couple times and I'm pretty certain that when your level gets higher, higher numbers of enemies show up, higher level enemies show up (never see visionaries in the castle when you first start, at higher levels they're freaking everywhere), and chests spit out more money.

So it would seem the best strategy is to concentrate on buying runes and equipment first, and only THEN start cranking up the manor so the difficulty doesn't outpace you.

just bought this on the summer sale. not sure if i'm enjoying. i've reached maya and darkness a few times but chickened out each time. still, it feels like traps are more abundent (more spike, turrets), more enemies (bullet hell). the slow sword swing is totally messing up my game.

No, difficulty does *not* scale with level!

The only thing that scales is the cost of runes and mansion upgrades. But that does not scale based on your level. It just gradually becomes more expensive as you buy more of them. Play more, level up and eventually you will breeze through it. You will complete it and reach NG+ in no time.

The idea of the gaining of levels and powerups is so that you can reach the more difficult parts of the castle. The difficulty is linear based on each area you visit, as opposed to the entire castle scaling with you.